Are you absolutely sure this needs replacement? Is there a broken piece missing? If not, like you already noted, the HA filters come in two pieces per bulb, so in effect, they are already "broken". That might not be an issue though, and unless there is a piece missing or the HA filter can no longer be held in place in this condition (IDK), I doubt one further breakage would influence its function.

Well, it fell out of the enlarger head when I opened it.

On the right side of the head, one of the two pieces is intact and the other split down the middle. The two parts were floating around inside the head.